1. Field of the Invention
This invention relates to rotary furnaces and, more particularly, to rabbles used therein for moving aggregate materials during thermolytic processing.
2. Description of Prior Art
Sealed rotary furnaces have long been used for drying or pyrolyzing various materials such as coal, coke or other aggregates. In a particular rotary furnace, externally generated hot gases are introduced into a lower furnace chamber and rise through a rotating perforated hearth, thereby heating materials carried on the hearth. For further discussion of this type of furnace, see U.S. Pat. Nos. 4,818,222 and 4,834,650.
In rotary furnaces, a rabble system is typically used to urge materials on the hearth from one location to another in response to relative rotation between the hearth and the rabbles, as disclosed in U.S. Pat. Nos. 3,475,286; 3,612,497; 3,740,184; 3,788,800; 3,859,172; and 4,149,845. The action of the rabbles on the aggregate materials, in addition to thermal or shearing stresses within the aggregate bed, results in breakdown of the materials and accumulation of "fines" on the perforated hearth below the underside of the rabbles. It has been found that accumulation of fines on the hearth causes blockage between the perforations in the hearth and the coarse aggregate materials on the hearth, resulting in an excessive pressure drop through the bed of materials. The build-up of fines around and under the rabbles has been found to extend throughout the hearth, blocking the flow of heating gas and retarding heating rates.
It is therefore an object of the present invention to provide a device for removing accumulated fines from the perforated hearth. It is a further object to relieve the burden of overlying coarse aggregate materials from the accumulated fines, allowing the fines to be lifted off the hearth in the upflowing heating gas. It is a still further object to replace the fines at a location where blockage between the hearth and the aggregate materials is avoided.